We have determined that gene sequences homologous to the DNA domain of SV40 large T antigen are present and expressed in uninfected, nontransformed cells. Expression of this gene appears to be cell growth-\and, in fact, cell cycle-related. This application proposes experiments designed to elucidate in greater detail: (1) the extent of homology to large T antigen at both the nucleic acid and amino acid level; (2) the expression of this gene during the various parts of the G-1 stage of the cell cycle; and (3) the role it plays by itself and in conjunction with other cell division cycle genes in the fundamental processes of cell proliferation. Such studies will provide important information about the nature and temporal sequence of events involved in the progression of a mammalian cell from [unreadable]G[unreadable]o through G1 to S and eventually on to mitosis. (N)